A level sensing apparatus is used to detect the level of liquid, liquid particles, or powdery particles in an industrial automatic system or a personal system. The level detected by the level sensing apparatus means the position or height of a charging surface (or filling surface) of a detected material and the level sensing apparatus is used to sense the height of the detected material charged (or filled) in a charging tank (or a filling tank). That is, the level sensing apparatus is applied to an oil tank of a vehicle to be used to detect a charging amount (or a filling amount) of oil and also used to detect the amount of an injected material charged in the charging tank and injected into a product in an automation factory.
The level sensing apparatus is classified into a float level sensing apparatus, an electronic level sensing apparatus, a wave level sensing apparatus, or a capacitance level sensing apparatus according to a driving method.
Among them, the capacitance level sensing apparatus senses the level of the detected material by using a capacitance increased as liquid, powdery particles, and the like having a dielectric constant larger than air ascend to be close to an electrode included in the level sensing apparatus.
Since the capacitance level sensing apparatus among them uses the dielectric constant of a charged material unlike the float level sensing apparatus, the electronic level sensing apparatus, the wave level sensing apparatus, and the like, the dielectric constant of the charged material serves as a primary factor in sensing.
However, in this case, the dielectric constant of the charged material may rapidly vary depending on temperature, which causes the level sensing apparatus to malfunction. For example, in the case of a level sensing apparatus that detects the height of water injected into a container, the dielectric constant of water significantly varies depending on temperature, as a result, a reference for compensating the varied dielectric constant is required.
Referring to FIG. 1 illustrating characteristics of water, the dielectric constant of water is maintained in the range of 75 to 80 at room temperature, but as temperature increases, the dielectric constant varies to approximately 50. Therefore, the difference between the dielectric constants of water which is the detected material is twice the maximum. In this case, even though water is charged up to the corresponding height, the dielectric constant of water decreases to one half, as a result, the level sensing apparatus cannot sense the height.
For this reason, in order to solve the problem, in the related art, a level sensing apparatus was used, which has a structure in which another level sensing apparatus for the reference is installed on the bottom of the container to acquire variation in the dielectric constant of water depending on variation of temperature on the basis of a capacitance detected from the reference and reflect the acquired variation in the dielectric constant when water starts to be charged from the bottom.